The proposed research project investigates the ways in which adolescent social relationships promote or inhibit the development of healthy adult family relationships. Three ideas rooted in the family literature of several disciplines motivate this project: 1) relationships early in life are socializing agents for later life social relationships; 2) adolescent relationships from various domains are important for later social relationships; and 3) content and quality of relationships, rather than mere existence, are important dimensions that condition the effects of early relationships on later relationships. The project will assess how adolescent social relationships affect the development and maintenance of young adults' family relationships with cohabiting partners, spouses, and young children. The analyses rely on data from four complementary sources: The National Longitudinal Study of Adolescent Health (Add Health); the Youth Development Study (YDS); the National Survey of Families and Households (NSFH); and the Minnesota Longitudinal Study of Parents and Children (MLSPC). The proposed development plan is designed to accomplish four objectives: 1) to develop a cross-disciplinary understanding of early life course relationships using ideas from demography, sociology and developmental psychology; 2) to expand the range and depth of my substantive and technical knowledge on the developmental significance of early social relationships; 3) to present new work at scholarly conferences and publish in leading journals; and 4) to prepare a grant proposal to support an independent research program. Training components include mentorship by leading family demography, life course and social relationship scholars, participation in workshops and seminars, and coursework in child psychology and methodology. This research and training proposal brings together insights from multiple disciplines to understand the contribution of a constellation of adolescent relationships on social relationship development into adulthood.